1. Field of the Invention
This invention relates generally to the formation of an improved silicon oxynitride material and to a photochemical vapor deposition process for forming this material.
2. Description of the Prior Art
In the fabrication of semiconductor integrated circuits and devices, a layer of silicon nitride (Si.sub.3 N.sub.4) is frequently used as a dielectric layer to provide electrical insulation between adjacent structures or circuits and prevent unwanted current flow therebetween. In addition, a silicon nitride layer is often used as a passivation layer on a device to prevent contamination of the device by air, moisture, mobile ions, and other impurities which might cause corrosion and degradation of device performance. Further a silicon nitride layer may be used as a mask during such device fabrication processes as ion implantation, ion diffusion, and metal deposition. In all such uses of silicon nitride, it is important to have a dielectric material with good electrical and morphological integrity, good hardness and scratch resistance, and good adhesion to the substrate.
Silicon nitride layers of good quality have been prepared by a conventional chemical vapor deposition reaction or thermal process, from silane and ammonia reactants at temperatures between 800.degree. and 100.degree. C., as described, for example, by J. A. Amick, G. L. Schnable, and J. L. Vossen, in the article entitled "Deposition techniques for dielectric films on semiconductor devices," in the Journal of Vacuum Science and Technology, Vol. 14, No. 5, Sept./Oct. 1977, at page 1059. However, in the fabrication of certain semiconductor devices, a high temperature produces undesirable side-effects, such as diffusion and boundary migration of existing structures on the device or decomposition of a compound semiconductor material, such as gallium, indium phosphide, or mercury cadmium telluride.
Consequently, it is frequently advantageous to use a low-temperature process for depositing a layer of silicon nitride on a temperature-sensitive device. Silicon nitride layers have been prepared at relatively low-temperature by using a photochemical reaction i.e. a chemical reaction which is induced or initiated by radiation, and such layers are referred to herein as "photonitride" layers. One process for depositing a Si.sub.3 N.sub.4 layer uses a photosensitized reaction between silane (SiH.sub.4) and hydrazine (N.sub.2 H.sub.4), as discussed by M. G. Collet, in the publication entitled "Depositing Silicon Nitride Layers at Low Temperature Using a Photochemical Reaction," in the Journal of the Electrochemical Society: SOLID STATE SCIENCE AND TECHNOLOGY, Vol. 116, No. 1, January 1969, pages 110-111 and in U.S. Pat. No. 3,620,827. In such a process, mercury (Hg) is excited by radiation of a particular wavelength (i.e. 2537 angstroms) to form mercury in an excited state (Hg*). The Hg* then collides with the SiH.sub.4 and the N.sub.2 H.sub.4 to cause these molecules to form radicals which then interact to form Si.sub.3 N.sub.4. Further studies of this process, as well as of a fast-flow system which uses a mixture of SiH.sub.4 and ammonia (NH.sub.3), are reported by C. H. J. v. d. Brekel and P. J. Severin, the articles entitled "Control of the Deposition of Silicon Nitride Layers by 2537 .ANG. Radiation," in the Journal of the Electrochemical Society: SOLID STATE SCIENCE AND TECHNOLOGY, Vol. 19, No. 3, March 1972, pages 372-376.
However, it was found that films prepared in accordance with the teachings referenced above had a high incidence of pinholes, which degraded the electrical integrity of the film and its electrical insulating properties. In order to improve upon these films, I co-discovered and developed a process for the preparation of low temperature photonitride films of good quality and containing minimal chemically bonded oxygen, as disclosed in U.S. Pat. No. 4,181,751 to T. C. Hall and J. W. Peters, assigned to the present assignee. By the process disclosed in the latter patent, a silane getter technique is used to remove oxygen and moisture from the nitrogen-containing reactants, in conjunction with a mercury vapor photosensitized reaction of a predetermined vapor mixture of silane, ammonia, and hydrazine at temperatures from 100.degree. C. to 300.degree. C. The silicon nitride layer formed by the above-described process was found to be substantially free of pinholes.
Furthermore, it was found that silicon nitride films prepared by the process of Collet described above typically exhibited poor scratch resistance and hardness characteristics, unacceptable adhesion, and irreproducible electrical properties, such as breakdown voltage, dielectric constant, and dissipation factor.
I have discovered that the material formed by the process of Collet is not a pure silicon nitride material, but rather a heterogeneous composition of silicon nitride and silicon oxynitride with free amorphous silicon incorporated therein. I have further discovered that the undesirable properties of this prior art silicon nitride material discussed above are directly related to the nature of its heterogeneous composition.
It is the alleviation of this prior art problem of the undesirable electrical and physical properties of a silicon nitride layer formed by photochemical vapor deposition to which the present invention is directed.